This invention relates generally to cathode ray tubes for color television and specifically to construction of improved color selection masks therefor.
Every commercial color television cathode ray tube today includes a color selection mask which allows a selected pattern of electrons to impinge upon a corresponding pattern of light-emitting phosphor elements deposited on a cathode ray tube panel. A typical color selection mask is of the shadow mask variety, comprising a thin sheet of steel having a pattern of electron-transmissive apertures etched therein. The apertures take various forms, although typically take the form of small round holes or vertical rows of aligned slots.
Naturally, it has been desirable to etch the mask apertures as accurately as possible in order to provide each mask with accurately positioned apertures having a predetermined size and shape.
The most straight-forward method of forming the mask apertures would be to coat one side of the mask blank with an etchant-resistant coating in which there is a pattern of holes through which an etchant may be applied to the blank. The blank would then be etched by spraying an etchant onto the portion of the blank surface not protected by the etchant-resistant coating. This spray would be continued until the etchant mills a hole through the blank. Since this method of forming mask apertures would include etching from only one side of the blank, it is referred to herein as one-sided etching. FIG. 1 is a portion of a color selection mask as it would appear if etched by such a one-sided etching process.
It has been found that it is difficult to control the size of the holes which are formed in a blank with one-sided etching. For example, variations in the thickness of a blank, which is normally made of 6 mil steel, etching non-uniformities and hole cleanliness tend to introduce undesirable variations in the size and shape of the mask apertures. A particularly undesirable characteristic of one-sided etching is that, while the etchant is creating an opening in the steel in the direction of the thickness of the blank, it is also laterally etching away or "undercutting" the metal of the blank beneath the etchant-resistant coating which lies on the surface of the blank. This undercutting continues as long as the etchant is applied to the blank and is definitely undesirable from the standpoint of mask strength. The thicker the blank, the longer the etchant must be applied to completely etch through the blank, thus a greater amount of undercutting is caused to occur during the process. Typically, an etchant will undercut or etch laterally about 0.5 mils for every 1 mil of through etching. Thus, for a 6 mil thick blank, undercutting may eat away up to 3 mils or more of the blank around each aperture.
In order to avoid the undesirably large amount of undercutting which is associated with one-sided etching, methods of etching masks from two sides have been developed and are used commercially throughout the world. Conventional methods of manufacturing color selection masks start with a flat blank sheet of metal nominally 6 mils thick. Normally the first step in the etching process is to coat at least one surface of the blank with a photoresistant material. In the two-sided manufacturing process both sides of the blank are coated with the photoresistant material.
Photoresist materials are of either the positive-working or negative-working type and are referred to as either positive or negative photoresists. A positive photoresist, on exposure to actinic light, undergoes changes which render it soluble in a developing solution which may be used to wash away the exposed photoresist. A negative photoresist, on exposure to actinic light, undergoes polymerization and becomes insoluble in an associated developing solution.
Although either type of photoresist may be used, for purposes of simplicity and clarity of description, the following background discussion deals exclusively with the use of a positive photoresist.
In the two-sided etching process, corresponding mask masters having predetermined related patterns of light-transmissive areas and the photoresist-coated blank are placed on a conventional lighthouse where an illumination pattern of electron-transmissive apertures is formed on each blank surface by exposing the photoresist coatings to actinic light transmitted through each respective mask master. The exposed areas, corresponding to the pattern of apertures, are solubilized and are washed away in a developing solution, baring the metal electrode surfaces.
An etchant is sprayed from a battery of etchant nozzles onto the opposed blank surfaces, etching those areas of the blank bared by the exposure and development processes. The etchant mills through the blank from both sides to form the desired electron-transmissive apertures in the blank, as shown in FIG. 2.
In addition to the through etching of the blank, it has been found that lateral etching or undercutting of the photoresist layer also occurs at a rate of approximately one-half the rate of the through etching; thus in a typical blank 6 mils thick the lateral etching would normally be approximately 3 mils, or 11/2 mil around the aperture periphery. However, since in the two-sided process the etchant etches through the blank from each side, the lateral etching is correspondingly reduced. Yet, because of this lateral etching, even though substantially reduced by the two-sided process, it is extremely difficult to control the shape of the aperture periphery. It has been found, in the manufacture of slot-type masks, to be very difficult to form tie bars which are acceptably narrow without unduly weakening the mask.
As said before, customarily aperture etching is performed on a flat sheet of electrically conductive metal. After the desired apertures have been etched, the etched mask is spherically or biradially formed to approximately the same contour as the cathode ray tube faceplate to which it is to be mated. During the forming process varying degrees of stress are put on the blank, causing many of the apertures, especially those on the mask edges, to be deformed. The forming process consists primarily of a stamping or drawing operation which by its nature causes different stresses to be put on individual masks and mask sections. Similarly, the positioning devices for holding the mask on the forming device do not position each individual mask in the same location, thus each resulting formed mask is different from all others, causing non-uniformity in the array of apertures from mask to mask.
These stresses and the non-uniformity of the aperture array from mask to mask virtually rule out the possibility of making color selection masks that are interchangeable each with all others in the assembly of a cathode ray tube of a given size and shape. The present state of the art compensates for the deformation variations from mask to mask by uniquely mating a given mask to a given cathode ray tube faceplate. Each mask is used as the stencil or master during the photoprinting of the phosphor screen on the cathode ray tube faceplate. Thus, any irregularities in the mask are duplicated in the screen patterns formed on the faceplate. Clearly, since each mask has different aperture deformations and a non-uniform array of apertures, random interchangeability of masks and faceplates is not possible with conventional tube manufacturing methods.
It is desirable in the interest of standardizing mask aperture patterns, to preform the mask blank prior to the etching process. Etching a preformed blank would virtually eliminate aperture deformation caused by the forming process. Substantial aperture pattern uniformity from mask to mask would enable the faceplate phosphor pattern to be made with a single mask master.
The primary disadvantage of the two-sided etching approach described above is the strict requirement of hole pattern alignment needed on opposite sides of the blank to precisely form properly sized and shaped apertures. This disadvantage has the practical effect of possibly limiting the applicability of the two-sided process to flat blanks. Thus, to manufacture masks from preformed blanks enabling random interchangeability of masks and cathode ray tube faceplates, it appears that some type of one-sided etching process, avoiding the requirement of registering masters on opposite sides of a pre-contoured blank, is the practical solution.
One approach to one-sided etching of the photoresist, described and claimed in U.S. Pat. No. 3,794,873, and having a common assignee herewith, utilizes a laminated color selection mask as shown in FIG. 3 wherein typical laminants comprise a thin nickel aperture-defining layer bonded to a relatively thick substrate layer of steel. Two etchants are used -- one to etch through the aperture-defining layer, hereafter referred to as an aperture layer etchant, and the other to etch through the steel substrate layer, hereafter referred to as a substrate etchant. It has been found that the severe undercutting of the aperture-defining layer which results from one-sided etching is somewhat reduced by this method. The substrate reacts quickly to the substrate etchant and slowly to the aperture layer etchant and the aperture-defining layer reacts similarly to the respective aperture etchant and substrate etchant. The respective reaction times reduce the overall etching time for etching through the blank, thereby reducing the time the etchant is in contact with the respective layers and resulting in reduced undercutting in the substrate. However, a serious problem still exists in the manufacture of slot-type masks where narrow tie bars (e.g., 4 mils or less) must be formed.
The severe undercutting associated with one-sided etching has led to alternative methods of one-sided etching for color selection masks. In one method, described and claimed in the referent application Ser. No. 466,102, to avoid this severe undercutting and the registration problems associated with the two-sided etching process, a laminated color selection mask blank is coated on one side with an etchant-resistant coating. A typical laminate mask blank may comprise a 1/2 mil aperture-defining layer of nickel, a 11/2 mil core layer of copper alloy, and a 4 mil steel substrate, as shown in FIG. 4. After the etchant-resistant coating is applied, an aperture array is made by exposing the photoresist coating, through a master, to actinic light in developing the exposed areas. An etchant, which may be ferric chloride, is then applied to the blank. The first etchant chemically etches the aperture-defining layer and the core layer. After a wash process, a second etchant, typically ferric sulfate, is applied to the partially etched mask blank. The nickel aperture-defining layer and the copper core layer, being impervious to ferric sulfate, resist etching while the steel substrate is being etched. Reduced undercutting results. This process of etching has formerly been labeled a laminated coring one-sided etching process.
Another method of performing one-sided etching without producing severe undercutting, described and claimed in the referent application Ser. No. 471,420, uses a laminate mask blank which may be composed of a 1/2 mil nickel aperture-defining layer bonded to a 6 mil substrate of steel. See FIG. 3 for basic construction of a blank used in this method. The color selection mask blank is first coated with a photoresist and etched with an etchant, typically ferric chloride. After the first etching process the blank, including the cavities etched by the first etchant, is again coated with a photoresist material (preferably of the positive-working type) and the cavity bottoms only are subsequently exposed to ultraviolet light. Upon development of the photoresist, the unexposed photoresist is retained on the surface of the mask blank and the undercut portions of the cavity, leaving exposed only that portion of the cavity exposed (on the second exposure) to the ultraviolet irradiation. Upon application of a second etchant such as ferric sulfate, the substrate is etched through. The resulting mask aperture profile is not shown, but is similar to that shown in FIG. 4. The same method may be used on a single layered blank, as shown in FIG. 5, a method where the etching is from the convex side of the preformed blank.
Each of the prior art methods of one-sided etching represent improvements in the basic one-sided aperture etching process, but have been found not to be completely satisfactory in every respect. The myriad of problems associated with one-sided etching, the most severe being the undercutting problem, has before this invention made one-sided etching a difficult approach to etching acceptable apertures of the slot type in a preformed mask blank. In a slot-type mask, the vertically running slats separating the openings or slots in the mask through which the electrons pass on their way to the cathode ray tube screen are held together by horizontally running tie bars which, for the sake of maximized brightness and minimum moire' pattern generation, should be as narrow as possible. The one-sided etching processes known to date are not completely acceptable because of the weakened condition of the mask which results from the undercutting or etching away of the tie bar areas. To compensate for the described undercutting, and to impart sufficient strength to the mask, the tie bars have had to be made undesirably wide. This invention provides a method of etching preformed blanks maintaining narrow, structurally strong tie bars having reduced undercutting of the aperture-defining layer.
The above discussion has dealt primarily with the problems associated with etching or shadow masks from one side and certain prior art attempts to cope with such problems, particularly in the context of shadow masks which are preformed (precontoured) before being etched (from one side). This invention has broader applicability, however.
In tubes of the negative guardband, black surround type, as explained, the electron beam landing spots are larger than the impinged phosphor elements. Since in conventional practice the shadow mask is used as the exposure stencil during the photoexposure operations used to screen the faceplate, some method must be provided for causing the electron beam spots to be larger than the impinged phosphor elements. Two methods are employed commercially. The first is the so-called "re-etch" or "etch-back" method wherein the shadow mask apertures are originally formed to the (smaller) size of the phosphor elements, and then after the screening operations, the shadow mask is "re-etched" (etched a second time) until the shadow mask apertures are larger than the phosphor elements by an allotted tolerance value, thus producing the desired negative guardband.
The second method used commercially to cause the mask apertures to be larger than the associated phosphor elements is to use a shadow mask which has full-sized apertures and, by the use, e.g., of special photoreduction techniques during the screening operation, to cause the phosphor elements to be smaller than the shadow mask apertures.
By the application of this invention, there may be provided yet another commercially practicable method for making shadow mask-faceplate assemblies in which the mask apertures are ultimately larger than the associated phosphor elements.